CHAPTER TWO 



The Visual Pigments and 

 Their Photoproducts 



In 1839 KROHN noted that the rods of Cephalopoda were red but, 

 so far as vertebrates are concerned, the first observations seem to be 

 those of HEiNRiCH MULLER (1851) on the frog. In 1856 muller 

 wrote The substance of rods is often seen to be reddish . . . if it be 

 of sufficient thickness ; as for instance when a rod is viewed on end, 

 or several are seen lying together, one over another. This colouring 

 is not uniform all over, but is sometimes stronger, sometimes weaker, 

 often unobservable ; and although it appears in eyes which are 

 quite fresh, it may perhaps depend on an imbibition of the colouring 

 matter of blood.' 



The first investigators laid stress on the need for fresh retinae to 

 demonstrate the red colour and were oblivious to its relations to 

 light. They had doubts whether the colour was due to a pigment 

 other than blood. Some even doubted whether the colour indicated 

 a pigment at all and thought it possible that the appearances were 

 due to interference phenomena. 



In 1876 BOLL discovered that the colour was destroyed by fight. 

 Lest the reader be surprised by the apparent tardiness of this observa- 

 tion we quote from KtJHNE (1878) that 'whoever has busied himself 

 with the retina wiU be reminded by boll's discovery (and thereby 

 receive a wholesome admonition of the fimits of his own ability), that 

 he has already seen something of the kind before. He will perhaps 

 remember that puzzling blood-clot — which at one moment he saw, or 

 thought he saw, under the retina, and which the next moment dis- 

 appeared. What he then passed over so lightly was nothing less than 

 the key of the secret, how a nerve can be excited by light. In other 

 words, it was the first fact disclosing the existence of photo-chemical 

 processes in the retina.' 



The speed with which the colour of an excised retina disappears 

 depends on the fight intensity. In artificial light the change may be 



25 



